Poison shyness is a behaviour whereby an animal which has ingested a toxic substance subsequently avoids ingesting that substance again. It occurs in nature where animals, usually generalists, have evolved the behaviour to avoid toxicosis. It is often observed in pest control when poisoned baits for insects and mammals are ingested at sublethal doses; the target species subsequently detect and avoid these poisoned baits. Animals may also exhibit bait shyness which occurs when the animal tastes a toxic bait, usually (although not always) becomes ill and then subsequently avoids the un-adulterated bait.[1] This learned behaviour is technically known as conditioned food aversion learning.[2] The terms "poison shyness" and "bait shyness" are sometimes used interchangeably.

Animals learn an association between the odour of a toxic substance and its toxicity. This allows them to detect and avoid the toxin when it is next encountered. In pest control, to prevent this, a special form of poison is used; the chemical is not immediately toxic and is odourless. When the poison is ingested, the active component is transported to the liver where it is oxidised to a toxic substance which then kills the animal. With sub-lethal doses of such a chemical, the animal cannot learn the association between the odour of the food and its toxicity, thereby preventing poison shyness from developing.[3]

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For any organism to survive, it must have adaptive mechanisms to avoid toxicosis. In mammals, a variety of behavioral and physiological mechanisms have been identified that allow them to avoid being poisoned. First, there are innate rejection mechanisms such as the rejection of toxic materials that taste bitter to humans. Second, there are other physiologically adaptive responses such as vomiting or alterations in the digestion and processing of toxic materials. Third, there are learned aversions to distinctive foods if ingestion is followed by illness.

Squirrel monkeys (Saimiri sciureus) and common marmosets (Callithrix jacchus) show one-trial learning with the visual cues of color and shape, whereas only the marmosets are able to do so with an olfactory cue. Both species showed a tendency for quicker acquisition of the association with the visual cues compared with the olfactory cue. All individuals from both species were able to reliably remember the significance of the visual cues, color and shape, even after 4 months. Illness for these species is not a necessary prerequisite for food avoidance learning but, presumably, the innate rejection responses toward highly concentrated but non-toxic bitter and sour tastes are sufficient to induce robust taste aversion learning and retention.[4]

Conditioned taste aversion has been widely used as a method of pest control and conservation. these aversions have been induced in both predator and prey species.[5]

In rats, bait shyness can persist for weeks or months and may be transferred to nontoxic foods of similar types.[6] There are indications that bait shyness in coyotes can be transferred from pieces of meat to a temporary aversion to live sheep.[7] Bait shyness may sometimes be location specific and not transferred to different localities.[8]

Crows: Conditioned taste aversion has been used to control crow (Corvus brachyrhynchos) predation on eggs - a problem for bird sanctuaries and farmers with outdoor chickens. The researchers put a sickness-causing agent in several eggs, painted them green and then placed them where crows could eat them. After eating the tainted eggs, the crows avoided eating green eggs. The crows subsequently avoided eating green eggs whether they contained toxin or not. The crows also continued to eat unpainted and non-toxic chicken eggs
[9][10] However, another study tested if Carrion Crow (Corvus corone) predation on Little Tern (Sterna albifrons) eggs could be decreased by conditioned taste aversion. The study failed to find an effect because the crows were able to distinguish treated eggs during handling, without consuming a significant amount of the illness-inducing compound.[11]

Quoll: In Australia, a critically endangered predator, the northern quoll (Dasyurus hallucatus) is threatened by the invasion of the highly toxic cane toad (Bufo marinus). Following toad invasion, quoll populations have become extinct across Northern Australia. A conditioned aversion to live toads in juvenile northern quolls was successfully established by feeding them a dead toad containing a nausea-inducing chemical (thiabendazole).[12]

Multiple predators: When surrogate eggs of the Sandhill Crane (Grus canadensis) were laced with an illness-producing substance, egg predation decreased in a location which contained multiple potential predators.[13]

Taste aversions have been developed in wolves, coyotes, and other canids to protect livestock and vulnerable wildlife.[14] In 1974, it was reported that coyotes (Canis latrans) learned to avoid hamburger after eating hamburger treated with Lithium chloride and could transfer such a drug-induced aversion of LiCl-tainted sheep or rabbit (Sylvilagus sp.) flesh to the corresponding live prey.[15] Olfactory (cologne) and visual (a red collar on the sheep) cues increased the suppresion of predation through conditioned learned aversion in coyotes, although this was for a limited duration.[16]

Grazing livestock frequently eat poisonous plants, and death often results. Behavioral adjustments by conditioned taste aversions may protect animals from over-ingestion of toxic plants. Three species of plants with different mechanisms of toxicity were tested for their ability to condition a taste aversion in sheep. Only woody aster conditioned a taste aversion indicating that conditioned aversions to selenium-containing plants help to deter consumption of such plants by grazing ruminants.[17]

The cultivation of woody plants (olive trees, grapevines, fruit trees, etc.) can benefit from having animals e.g. sheep and goats, grazing the same area as their faeces nourish the soil thereby reducing the use of herbicides and fertilisers. However, these same animals sometimes eat the crops. Lithium chloride has been used to develop conditioned taste aversion to olive leaves and shoots in sheep and goats.[18]